A Thermal Origin of LLSVP Seismic Signals? Consequences of Perovskite Phase Mixing in the Deep Lower Mantle

Abstract The Earth's lower mantle hosts bridgmanite (Bdm) and davemaoite (Dvm) which can potentially mix into a single Bdm‐Dvm phase. The favorability and elastic consequences of such mixing are still disputed meaning that we currently cannot search for it in the seismic record of the Earth. In this paper we use ab initio models of stability and elasticity to predict that such mixing both occurs and has detectable seismic consequences. We predict that in a pyrolitic mantle with ∼300K excess heat, perovskite phase mixing will produce sharp sided (<90K) regions of around 830 km height with V s decreases of ∼−2% and density reductions of −0.25%–−1%. All three of these properties match previously observed properties of large low shear velocity provinces (LLSVPs) suggesting a possible origin of these signals that is purely due to thermally induced phase mixing. While other explanations are possible our results demonstrate that perovskite phase mixing is thermodynamically favored in hot pyrolitic regions of the lowermost mantle and produces seismic anomalies of the same order as those observed in LLSVPs. Consequently, models of LLSVP origin and dynamics must account for this effect which reduces the need for steep thermal or chemical gradients to produce LLSVP seismic signals. We also predict that perovskite mixing behavior will be entirely suppressed in basaltic mixtures allowing a clearer separation between the elasticity of pyrolitic and basaltic compositions.